Metal members installed in an open circulating cooling water system, for example, a heat exchanger, a reaction tank, and a pipe made of carbon steel, copper, or a copper alloy become corroded by coming in contact with cooling water. Therefore, there is generally provided an anti-corrosion treatment by addition of a chemical agent.
In order to suppress corrosion of the heat exchanger, the reaction tank, and the pipe made of carbon steel, there is being added to the cooling water a phosphorous compound such as an orthophosphoric acid salt, a hexametaphosphoric acid salt, a hydroxyethylidene phosphonic acid salt, a phosphonobutane tricarboxylic acid salt, and the like. There are also cases where a heavy metal salt such as a zinc salt and a bichromate salt are added solely or in a combination.
Particularly, in water quality of low calcium hardness, an anti-corrosion effect due to a calcium hardness component cannot be expected and, therefore, the metal members are easier to be corroded than in water quality of high calcium hardness. Especially when the calcium hardness is 100 mg CaCO3/L or less, a certain amount or more of an anti-corrosion agent such as a phosphoric acid salt and the like needs to be added, and a low molecular weight polymer for dispersing the anti-corrosion agent needs to be added in a high concentration (see Kurita Water Industries' Chemicals Handbook Editing Committee (Eds.), “Kurita Water Industries' Chemicals Handbook (4th ed.),” Entry 178, “7.3.1 Prevention of Corrosion of Carbon Steel, Figure 7.29.”). However, this method for treating water is contrary to a trend of recent years to reduce environmental loads.
For this reason, when water of low calcium hardness is used as makeup water for the cooling water, there has been desired a polymer for dispersing the anti-corrosion agent, which will enable the anti-corrosion agent to exhibit a high anti-corrosion effect at a lower concentration.
As a treatment method with a reduced environmental load in a water quality condition where the calcium hardness is low and metal members are easily corroded, there have been proposed methods for improving anti-corrosion effects by means of adjusting water quality components.
For example, in Patent Literature 1, there is disclosed a method for suppressing corrosion of metal in an open circulating cooling water system, comprising: adjusting water quality so that a Langelier index becomes 1.5 or more, and a product of a silica concentration and calcium hardness becomes 2000 or more; and adding a copolymer of maleic acid and isobutylene.
In Patent Literature 2, there is disclosed a method for suppressing corrosion of metal comprising adding, in a specific ratio, a maleic acid-based polymer A and a copolymer B of maleic acid and the like and a nonionic mono-ethylene-based unsaturated monomer to a water system which has been adjusted so that a Langelier index becomes 1.5 or more, and a product of a silica concentration and calcium hardness becomes 2000 or more.
In Patent Literature 3, there is disclosed a method for suppressing corrosion of metal in a water system, comprising: adding a phosphoric acid salt, a zinc salt, and an M alkalinity component; and adjusting respective concentrations of total phosphoric acid and total zinc to 1 mg/L or less, and a Langelier index at 30° C. to 1.2 or more.
However, in water quality of low calcium hardness, all methods of the Patent Literatures 1 to 3 need addition of an enormous amount of a chemical agent in order to adjust the calcium hardness, the silica concentration, and the Langelier index to certain values.
On the other hand, as a polymer which shows a high scale inhibiting effect and a high anti-corrosion effect in a water system of high calcium hardness, Patent Literature 4 discloses a (meth)acrylic acid-based polymer having a sulfonic acid group at a main chain terminal, whereby, it is described, gelation resistance performance is improved and an excellent anti-corrosion effect is exhibited even in a water system of a high calcium concentration. However, the Patent Literature 4 does not disclose anything about an anti-corrosion effect in a water system of low calcium hardness which has a high tendency to corrode.